1. Field of the Invention
The present invention relates to an electroconductive endless belt (hereinafter also simply referred to as “belt”). The endless belt is used when a toner image is transferred to a recording medium such as paper in an electrostatic recording process performed in an electrostatic recording apparatus or an electrophotographic apparatus such as a copying machine or a printer. The toner image is formed by supplying a developer onto the surface of an image-forming member such as a latent image bearing member bearing a latent image thereon.
2. Description of the Related Art
In an electrostatic recording process performed typically in a copying machine or a printer, printing is performed by the steps of uniformly electrifying the surface of a photosensitive member (latent image bearing member), forming an electrostatic latent image by projecting an optical image from an optical system onto this photosensitive member to diselectrify the area to which light is applied, then supplying toner to this electrostatic latent image to form a toner image by electrostatic adhesion of the toner, and transferring the toner image to a recording medium such as paper, transparent paper for overhead projector use, or photographic paper.
Also in a color printer or color copying machine, the printing is fundamentally performed in accordance with the process described above. However, a color printing process uses four color toners, magenta, yellow, cyan, and black for reproducing a color tone and further includes a step of overlapping the color toners at a predetermined ratio. Various methods have been proposed in order to execute this step.
Such methods include, for example, image-on-image development method as a first category. In this method, the above four color toners, magenta, yellow, cyan, and black, are sequentially supplied onto a photosensitive member so as to be superimposed for development in order to convert an electrostatic latent image into a visible color toner image, as in monochromatic printing. An apparatus according to this technique can have a relatively small size. However, it is very difficult to control the gradation, and as a result, a high quality image may not be obtained.
A second category is a tandem system using four photosensitive drums. In this method, four photosensitive drums are aligned; latent images on the drums are developed by respective color toners, magenta, yellow, cyan, and black to form four toner images of magenta, yellow, cyan, and black; the above respective toner images on the aligned photosensitive drums are then sequentially transferred to a recording medium, such as paper, for superimposing the images thereon and thereby reproducing a color image. By this method, superior images can be obtained; however, the apparatus becomes large and expensive, because the four drums each provided with an electrification mechanism and a development mechanism are aligned.
FIG. 2 shows an example of a printing portion of a tandem image-forming apparatus. Four printing units are provided for respective yellow Y, magenta M, cyan C, and black B toners. The printing units each include a photosensitive drum 1, an electrification roller 2, a developing roller 3, a developing blade 4, a toner supply roller 5, and a cleaning blade 6. The toners are sequentially transferred onto paper transported by a transfer and transport belt 10 which is circularly driven by a drive roller (drive member) 9, thereby forming a color image. Electrification and diselectrification of the transfer and transport belt 10 are performed by an electrification roller 7 and a diselectrification roller 8, respectively. The apparatus further includes an attraction roller (not shown) for electrification of paper to attract it by the belt. By the structure described above, the generation of ozone can be suppressed. The attraction roller transfers paper from a transport path onto the transfer and transport belt 10 and also fixes it thereon by electrostatic attraction. In addition, a transfer voltage is decreased after the transfer to decrease an attraction force between paper and the transfer and transport belt 10 so that paper can be separated from the transfer and transport belt only by means of self stripping.
Materials for the transfer and transport belt 10 include a resistive material and a dielectric material; however, each material has advantages and disadvantages. Since a resistive belt retains charges for a short period of time when being used for transfer operation of the tandem system, charge injection caused by the transfer is low, and even by continuous transfer operation of the four colors, the increase in voltage is relatively small. In addition, even when being used repeatedly for the following paper, the resistive belt releases charges, and electrical reset is not required. However, since the electrical resistance of the resistive belt varies with the change in environmental conditions, the transfer efficiently varies, and/or the thickness and the width of paper adversely affect the transfer performance.
In contrast, a dielectric belt is not so configured to release injected changes spontaneously and is thereby configured to electrically control injection and release of charges. However, attraction of paper is reliably performed, and highly precise paper transport can be performed, because the dielectric belt can stably retain charges. In addition, the dielectric constant less varies depending on temperature and humidity, and a relatively stable transfer process may be performed in various environments. As disadvantages, the increase in transfer voltage may be mentioned which is caused by accumulation of charges in the belt as the transfer is repeatedly performed.
A third category is a transfer drum method. In this method, a recording medium such as paper is wound around a transfer drum, and the drum is allowed to rotate four times. During this rotation, magenta, yellow, cyan, and black toners provided on photosensitive members are sequentially transferred on the medium at respective rotations of the drum, thereby reproducing a color image. According to this method, a relatively high quality image can be obtained. However, when a thick recording medium such as a postcard is used, it is difficult to wind the medium around the transfer drum, and the type of recording medium is disadvantageously limited.
In addition to the image-on-image development method, the tandem system, and the transfer drum method, an intermediate transfer system has been proposed as a method in which a high image quality can be obtained, the size of the apparatus is not particularly increased, and the type of recording medium is not particularly limited.
That is, according to this intermediate transfer system, an intermediate transfer member is provided which is composed of a belt and drums designed to temporarily retain toner images transferred from respective four photosensitive members, and four photosensitive members having a magenta toner image, a yellow toner image, a cyan toner image, and a black toner image are disposed around this intermediate transfer member. In the structure described above, the four color toner images are sequentially transferred onto the intermediate transfer member to form a color image thereon, and this color image is then transferred onto a recording medium such as paper. Accordingly, a high image quality can be obtained, because the gradation is adjusted by superimposing the four toner images. The size of the apparatus is not particularly increased, because the photosensitive members are not necessarily aligned, unlike the tandem system. The type of recording medium is therefore not specifically limited, because the recording medium is not required to be wound around the drum.
FIG. 3 shows an image-forming apparatus using an endless belt as the intermediate transfer member by way of example of an apparatus forming a color image in accordance with the intermediate transfer system.
The apparatus shown in FIG. 3 includes a drum-shaped photosensitive member 11 which is allowed to rotate in the direction shown by the arrow in FIG. 3. The photosensitive member 11 is electrified by a primary electrifier 12, a part of the member 11 exposed to an image exposure 13 is then diselectrified thereby, an electrostatic latent image corresponding to a first color component is subsequently formed on the photosensitive member 11, the electrostatic latent image is further developed by a developer 41 using a magenta toner M which is the first color, and as a result, the first-color magenta toner image is formed on the photosensitive member 11. Next, this toner image is transferred onto an intermediate transfer member 20 circularly driven by a drive roller (drive member) 30 while it is being in contact with the photosensitive member 11. In this case, the transfer from the photosensitive member 11 to the intermediate transfer member 20 is performed at a nip portion formed therebetween by a primary transfer bias applied from a power source 61 to the intermediate transfer member 20. After the first-color magenta toner image is transferred onto this intermediate transfer member 20, the surface of the photosensitive member 11 is cleaned by a cleaning device 14, and a first development and transfer operation of the photosensitive member 11 is complete. Subsequently, while the photosensitive member 11 is allowed to rotate three times, at the respective rotations, a second-color cyan toner image, a third-color yellow toner image, and a fourth-color black toner image are sequentially formed in that order on the photosensitive member 11 at the respective rotations by sequentially using developers 42 to 44. Thus, the four color images are superimposed on the intermediate transfer member 20 at the respective rotations, and a composite color toner image corresponding to an object color image is formed on the intermediate transfer member 20. In the apparatus shown in FIG. 3, at the respective rotations of the photosensitive member 11, the positions of the developers 41 to 44 are changed so that development of magenta toner M, cyan toner C, yellow toner Y, and black toner B are sequentially performed.
Next, a transfer roller 25 is then brought into contact with the intermediate transfer member 20 provided with the composite color toner image thereon, and to a nip portion therebetween, a recording medium 26 is supplied from a paper feed cassette 19. At the same time, a power source 29 applies a secondary transfer bias to the transfer roller 25, and the composite color toner image is transferred from the intermediate transfer member 20 onto the recording medium 26, followed by heating and fixing, thereby forming a final image. After the composite color toner image is transferred onto the recording medium 26, the intermediate transfer member 20 is processed by a cleaning device 35 so as to remove residual toners remaining on the surface and is then placed in a standby state for another image formation.
An intermediate transfer system as a combination between the tandem system and the intermediate transfer system has also been proposed. FIG. 4 shows an image-forming apparatus in accordance with an intermediate transfer system by way of example. In the method, color image formation is performed using an endless belt-shaped intermediate transfer member.
In the apparatus shown in FIG. 4, a first, second, third, and fourth development portions 54a, 54b, 54c, and 54d are sequentially disposed along an intermediate transfer member 50 for developing electrostatic latent images on photosensitive drums 52a, 52b, 52c, and 52d using yellow, magenta, cyan, and black toners, respectively, and this intermediate transfer member 50 is circularly driven in the direction indicated by the arrow shown in FIG. 4, so that four color toner images formed on the photosensitive drums 52a to 52d of the respective development portions 54a to 54d are sequentially transferred on this intermediate transfer member 50 to form a color toner image thereon. Subsequently, the formed toner image is transferred onto a recording medium 53 such as paper by transfer, thereby performing printout. In any apparatus described above, arrangement order of toners to use for the developing is not specifically limited and can be selected appropriately.
The apparatus shown in FIG. 4 further includes a drive roller or a tension roller 55 configured to circularly drive the intermediate transfer member 50; a recording medium feed roller 56; a recording medium feed device 57; a fixing device 58 configured to fix an image on the recording medium typically by heating; and a power source device (voltage application means) 59 configured to apply a voltage to the intermediate transfer member 50. The power source device 59 is configured to change the application direction of the voltage between the case where the toner image is transferred onto the intermediate transfer member 50 from the photosensitive drums 52a to 52d and the case where the toner image is transferred from the intermediate transfer member 50 to the recording medium 53.
Heretofore, such an electroconductive endless belt for use as the transfer and transport endless belt 10 or the endless intermediate transfer member 20 or 50 has generally been a semiconductive resin film belt or a fiber-reinforced rubber belt. Examples of the resin for use in the resin film belt include polycarbonate (PC) mixed with carbon black, polyalkylene terephthalate-based resins, and thermoplastic polyimide-based resins.
Japanese Unexamined Patent Application Publication No. 6-93175 discloses, for the purpose of providing a flame-resistant resin having high thermal stability, a flame-resistant seamless belt for use in intermediate transfer that is composed of polycarbonate as a main component, a polyalkylene terephthalate, carbon black, a flame retardant, an antimony compound, and a polyolefin. Japanese Unexamined Patent Application Publication No. 10-237278 discloses, for the purpose of providing an electroconductive seamless belt having high flexibility, a seamless belt composed of a thermoplastic resin combining a polyester, a polyester elastomer, an electroconductive filler, and a bromine-containing flame retardant. Japanese Unexamined Patent Application Publication No. 5-213504 discloses, for the purpose of providing a flame-resistant, highly durable electroconductive seamless belt, a seamless belt having a layer containing carbon black and a flame retardant and a durable outer layer containing carbon black and no flame retardant.
However, as disclosed in the above-mentioned patent documents, in known flame-resistant belts containing a flame retardant, the dispersibility of the flame retardant has never been investigated and is insufficient. In particular, in an example in Japanese Unexamined Patent Application Publication No. 10-237278, the dispersion of a flame retardant, a TBBA carbonate oligomer, in polybutylene terephthalate is insufficient, and the resulting belt is likely to have poor surface properties. Insufficiently dispersed particles of a flame retardant form a granular structure on a belt, causing image defects. Excellent dispersion of a flame retardant in a resin component is therefore important and is desired. The belt disclosed in Japanese Unexamined Patent Application Publication No. 6-93175 is mainly composed of a non-crystalline resin, polycarbonate, and has low durability, as indicated by the hinge property (fracture characteristic) of only several hundred times evaluated in an example. Thus, the belt cannot satisfy required characteristics.